A superionic material is a multi-component solid with simultaneous characteristics of both a solid and a liquid. Above a critical temperature associated with a structural phase transition, one of the atomic species in the material will exhibit liquid-like ionic conductivity and dynamic disorder within the rigid crystalline structure of the other. Discovered by Michael Faraday almost 200 years ago, superionic materials today hold promise for use in resistive switching devices, which follow from their abrupt changes in ionic mobility, as well as for use as safe, efficient solid-state electrolytes for rechargeable batteries. However, the fundamental processes and atomistic pathways determining how these materials switch and the new functional properties that emerge at the nanoscale are largely unknown and unexplored. Researchers have recently carried out a range of ultrafast x-ray spectroscopy and scattering experiments at three x-ray light sources, including the ALS, to obtain an atomic-level, real-time view of the transition state in superionic copper sulfide (Cu2S) nanocrystals.

Solid-State Ionics

At the dawn of the Information Age, the development of solid-state electronics represented a major upgrade over the use of mechanical switches and vacuum tubes. Devices incorporating solid-state electronics could be smaller, lighter, faster, more efficient, and more physically robust because electrons no longer had to travel through a vacuum in fragile glass tubes to create or process electrical signals. Instead, they moved within a solid material, and logic switches could be engineered by exploiting the physical properties of the material itself.

Now, with tools specifically optimized for the study of ultrafast phenomena in solids at the atomic scale, scientists are looking forward to reaping the technological benefits of the development of solid-state ionics, in which ions rather than electrons are the mobile species. For example, ion transport is a key step in moving charges between electrodes when recharging a lithium-ion battery. At present this is done using liquid electrolytes that are flammable—a serious limitation, especially for applications in cars or planes. More broadly, these studies demonstrate new means of using light to control the emergence of new phases and the speed at which these transformations occur. Future work may enable devices taking advantage of these ultrafast transformations for the development of resistive switching devices operating at the few-picosecond level.

Depiction of the copper sulfide superionic state with copper in blue and sulfur in yellow. Image credit: Greg Stewart/SLAC National Accelerator Laboratory.

In copper sulfide, the sulfur ions form the rigid crystalline cage and the copper ions occupy the interstices. Upon heating to 103 °C (217.4 °F), the sulfur cage expands into a largely vacant hexagonal structure, permitting the copper ions to hop from one local sweet spot to another within the fixed sublattice (there are many more available sites than copper ions).

Recent work at the nanoscale has shown that the transition temperature is reduced at small sizes and that polymer surface treatments can stabilize the superionic phase at room temperature. In addition, experiments on superionics for nanoscale resistance switching devices have observed switching on 100-microsecond time scales. Despite the long history of investigations into superionic materials, however, the mechanism of the structural reorganization and how that gives rise to high ionic conductivity remains largely unknown, especially at the nanoscale. Now, femtosecond studies carried out at the ALS, spectroscopic studies done at the Stanford Synchrotron Radiation Lightsource (SSRL), and diffraction studies performed at the Advanced Photon Source (APS) capture for the first time the atomic-level dynamics of a superionic nanocrystal as it transforms.

The research team looked at copper sulfide nanodiscs (10 nm in diameter) using ultrafast x-ray pump–probe techniques. The samples were photoexcited with 400-nm laser pulses then probed using synchronized femtosecond soft x-ray pulses generated at ALS Beamline 6.0.2 using established femtoslicing techniques. The resulting near-edge x-ray absorption fine-structure (NEXAFS) spectra at the copper L-edge revealed photo-induced changes in the local bonding geometry around the copper ions. By subtracting the laser-off spectrum from the laser-on spectrum (at a probe delay time of 400 ps), the team obtained a differential spectrum with features corresonding to a subtle redshift (i.e., a lowering of the energy) of the copper L-edge, which the researchers conclude corresponds to a lengthening of the copper bonds within the sample.

The researchers also recorded the near-edge absorption as a function of time to measure the rise time of the spectral redshift. A single-exponential fit to the data revealed an approximately 20-picosecond onset time for the spectral evolution. This observed time scale is comparable to the time it takes for a single ionic hopping event to occur. The measurements therefore indicate that the intrinsic time scale for the superionic phase transformation is governed by the ionic hopping time in these systems, identifying a transition state in which diffusional motion of the ions leads to the formation of the superionic crystallographic phase. Moreover, these results demonstrate that the transition can occur on picosecond time scales, opening up the possibility of nanoscale superionic-based switching devices controllable by light and operating at speeds orders of magnitude faster than previously demonstrated.

Left: The differential energy spectrum observed at a probe delay of t = 400 ps (black), with the unexcited spectrum overlaid (blue). The features of the differential spectrum correspond to a redshift of the copper L-edge. Right: The time-resolved change in the copper L-edge absorption strength at 935 eV with 200-femtosecond resolution. A single-exponential fit to the data (blue line) reveals a ~20-picosecond observed switching time, associated with the formation of the superionic phase.